Thermal spraying, also known as flame spraying, involves the heat softening of a heat fusible material such as metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. A thermal spray gun is used for the purpose of both heating and propelling the particles. In one type of thermal spray gun, the heat fusible material is supplied to the gun in powder form. Such powders are typically comprised of small particles, e.g., between 100 mesh U.S. Standard screen size (149 microns) and about 2 microns. The carrier gas, which entrains and transports the powder, can be one of the combustion gases or an inert gas such as nitrogen, or it can be simply compressed air.
Especially high quality coatings of thermal spray materials may be produced by spraying at very high velocity. One type of high velocity powder spray gun is disclosed in U.S. Pat. No. 4,416,421 (Browning). This type of gun has an internal combustion chamber with a high pressure combustion effluent directed through an annular opening into the constricted throat of a long nozzle. Powder is fed axially within the annular opening into the nozzle chamber to be heated and propelled by the combustion effluent.
Another high velocity thermal spray gun is disclosed in U.S. Pat. No. 4,865,252 (Rotolico et al). The gun has a nozzle member and a gas cap with a combustion chamber extending therefrom. A combustible mixture is injected coaxially into the combustion chamber at a pressure therein of at least two bar above atmospheric pressure. An annular outer flow of pressurized non-combustible gas is injected adjacent to the cylindrical wall of the gas cap. Powder in a carrier gas is fed axially from the nozzle into the combustion chamber, and an annular inner flow of pressurized gas is injected into the combustion chamber coaxially between the combustible mixture and the powder-carrier gas.
Various configurations have been used for mixing the combustion and oxygen gases in thermal spray guns. A siphon plug used in the gun of the above-mentioned U.S. Pat. No. 4,865,252 is typical. However, unless proper lighting and running procedures are followed, a siphon plug sometimes has a propensity to allow backfire at very high pressures utilized for high velocity, particularly with hydrogen fuel.
Other high velocity thermal spray guns are disclosed in U.S. Pat. No. 2,920,001 (Smith et al) and European Patent Application No. 0 361 709 A1. The former shows several early concepts of such guns. The latter shows an elongated gas cap extending from a nozzle that injects combustion gas and oxygen separately for mixing and combustion in the gas cap. Mixing within the nozzle of a low velocity gun is taught in U.S. Pat. No. 4,363,443 (Heuhne).